The present invention relates to a constant-voltage power supply circuit, and more particularly, to a constant-voltage power supply circuit provided with an overcurrent protection function.
A power supply circuit for supplying a constant-voltage power supply to devices such as a microprocessor unit (MPU) includes an overcurrent protection circuit. The overcurrent protection circuit prevents output of an overcurrent, which is generated when an electrical or mechanical failure occurs in a load. Such a constant-voltage power supply circuit is required to supply stable constant-voltage power while preventing generation of an overcurrent.
FIG. 7 is a schematic circuit diagram of a conventional constant-voltage power supply circuit 100. When an external power supply V1 goes on, a differential amplifier 1 is activated, and an output voltage of the differential amplifier 1 is applied to the gate of an output transistor T1, which is configured by a P-channel MOS (metal oxide semiconductor) transistor.
When the output transistor T1 goes on and an output voltage Vout is output, the output voltage Vout is divided by feedback resistors R1 and R2 to generate divided voltage at node N1. The divided voltage is applied to a non-inversion input terminal of the differential amplifier 1. A reference voltage V2 is applied to an inversion input terminal of the differential amplifier 1.
When the output voltage Vout increases, the potential at the node N1 also increases. This increases the output voltage of the differential amplifier 1. As a result, the drain current of the output transistor T1 decreases and the output voltage Vout decreases. When the output voltage Vout decreases, the potential at the node N1 also decreases. This decreases the output voltage of the differential amplifier 1. As a result, the drain current of the output transistor T1 increases and the output voltage Vout increases. Through such operations, the output voltage Vout converges on a constant voltage that is set based on the reference voltage V2.
Some of the drain current of the output transistor T1 is supplied to the collector of an NPN transistor T2 and the bases of transistors T2 and T3 via a resistor R3. The transistors T2 and T3 execute a current mirror operation.
A P-channel MOS transistor T4, which is operated based on the output voltage of the differential amplifier 1, executes a current mirror operation with the output transistor T1. The size of the transistor T4 is smaller than the size of the output transistor T1.
The drain current of the transistor T4 is supplied to the collectors of the transistor T3 and an NPN transistor T5 and to the bases of the NPN transistor T5 and an NPN transistor T6. The transistors T5 and T6 execute a current mirror operation. The collector of the transistor T6 is connected to a resistor R4 and to the gate of a transistor T7.
When the transistor T6 is turned on and a collector current flows through the transistor T6, the gate voltage of the P-channel MOS transistor T7 is lowered by the resistor R4. As a result, drain current flows through the transistor T7. This increases the gate voltage of the output transistor T1. As a result, the drain current of the output transistor T1 decreases.
In the constant-voltage power supply circuit 100 during normal operation, the operation of the feedback resistors R1 and R2 and the differential amplifier 1 keeps the output voltage Vout constant while changing the output current Iout of the output transistor T1. In this state, the drain current of the transistor T4 is entirely absorbed as the collector current of the transistor T3. This keeps the transistors T5, T6, and T7 off.
When the output current Iout increases and reaches a predetermined overcurrent detection value I1, the drain current of the transistor T4 cannot be further absorbed by the transistor T3. Thus, the transistors T5 and T6 are turned on. This turns on the transistor T7 and increases the gate voltage of the output transistor T1, decrease the output current Iout, and decreases the output voltage Vout. When the output current Vout decreases, the transistors T2 and T3 are turned off. Thus, the drain current of the transistor T4 keeps the transistors T5, T6, and T7 on, and the gate voltage of the output transistor T1 further increases.
As shown in FIG. 8, the output current Iout gradually decreases after the output current Iout reaches the overcurrent detection value I1. This gradually decreases the output voltage Vout. A predetermined restriction current I2 is continuously output even after the output voltage Vout reaches 0 V. In this way, the output current Iout decreases after reaching the overcurrent detection value I1. This control prevents the load from being damaged by an overcurrent.
In the constant-voltage power supply circuit 100, the restriction current I2 is continuously output even after the output voltage Vout decreases to 0 V. In this state, when the load current decreases, the drain current of the transistor T1 decreases and the drain current of the transistor T4 decreases. This decreases the base currents of the transistors T5 and T6. As a result, the drain current of the transistor T7 decreases and the gate voltage of the output transistor T7 decreases and the output current Iout increases. This increases the output voltage Vout and increases the base currents of the transistors T2 and T3. Thus, the drain current of the transistor T4 is absorbed by the transistor T3. As a result, the transistors T5, T6, and T7 are turned off so that the constant-voltage power supply circuit 100 recovers to a state in which constant voltage Vout can be output.